Optical fiber connectors are an essential part of substantially any optical fiber communication system. For instance, such connectors may be used to join segments of fiber into longer lengths; to connect fiber to active devices such as radiation sources, optical amplifiers, detectors and repeaters; or to connect fiber to passive devices such as switches and attenuators. The central function of an optical fiber connector is the maintenance of two optical fiber ends such that the core of one of the fibers is axially aligned with the core of the other fiber; and consequently, substantially all of the light from one fiber is coupled to the other fiber. This is a particularly challenging task because the light-carrying region (core) of an optical fiber is quite small. In singlemode optical fibers the core diameter is about 8 microns where 1 micron=1 .mu.m=10.sup.-3 mm. Another function of the optical fiber connector is to provide mechanical stability and protection to the junction in its working environment. For most optical connectors, achieving low insertion loss in coupling two fibers is generally a function of the transverse alignment of the fiber ends, the longitudinal gap between the ends, and the optical surface condition and end face geometry of either or both ends. Stability and junction protection is generally a function of connector design (e.g., minimization of the different thermal expansion and mechanical movement effects). An optical fiber connector typically includes a small capillary cylinder with a glass or plastic fiber installed along its central axis. This cylinder is interchangeably referred to as a ferrule or a plug.
In a connection between a pair of optical fibers, a pair of ferrules are butted together--end to end--and light travels from one to the other along their common central axis. In this conventional optical connection, it is highly desirable for the cores of the glass fibers to be precisely aligned in order to minimize the loss of light (insertion loss) caused by the connection; but as one might expect, it is presently impossible to make routine perfect connections. Manufacturing tolerances may approach "zero," but practical considerations such as cost, and the fact that slight misalignment is tolerable, suggest that perfection in such matters may be unnecessary.
One known design of an optical fiber connector is shown in U.S. Pat. No. 4,793,683; and its basic components comprise a precision molded plastic conical plug having an optical fiber centered therein, a compression spring disposed about a cylindrical portion of the plug, and a retention collar surrounding the plug and spring. The collar includes external threads that enable it to couple with another connector via a fixture having a precision molded alignment sleeve whose shape is best described as "biconic." This design has been superseded by the connector shown in U.S. Pat. No. 4,934,785 which comprises a cylindrical plug, a base member that holds the plug, a compression spring, and a cap that surrounds the plug and spring. In this design, only the cylindrical plug needs to be of high precision and is typically made from a ceramic material. When joining two of these plugs together, an alignment sleeve is used which comprises a split, thin-walled cylinder made of metal, ceramic or even plastic material. This alignment sleeve need not be as precise as the above-described biconic alignment sleeve.
Another known design of an optical fiber connector is disclosed in U.S. Pat. No. 5,212,752 (hereinafter the '752 patent). The optical connector comprises a ferrule assembly that includes a ferrule portion having a passageway for an optical fiber and a plug frame in which the ferrule assembly is disposed. Once the ferrule assembly has been disposed in the plug frame, the plug frame is assembled within another portion of the optical connector called a grip. The plug frame may be assembled within the grip in a plurality of rotational orientations with respect to the grip in such a way that the direction of eccentricity is aligned with a key of the grip. Once the plug frame has been coupled within the grip, the optical connector may be inserted into a coupling housing. The coupling housing is configured to allow two identical optical connectors to be inserted therein to provide an optical connection between two optical fibers terminated by ferrule assemblies within the optical connectors.
One of the advantages of the optical connector disclosed in '752 patent is that when the plug frame is inserted within the grip, the optical connector is provided with good side-loading characteristics due to the design of the grip and the manner in which the plug frame couples with the grip. One of the disadvantages associated with this optical connector is that, once the grip is installed, it cannot be removed. This is a disadvantage if, for some reason, tuning must be re-adjusted. The coupling housing is adapted to receive the grip. Although it may be possible to insert the plug frame into the coupling housing even when the plug frame is not disposed within the grip, removing the plug frame from the coupling housing once it has been inserted would be difficult, if not impossible without a special tool, due to the fact that there is no mechanism for detaching the plug frame from the coupling housing once it has been inserted. Furthermore, if the plug frame is not disposed within the grip, the side-loading characteristics of the optical connector are diminished.
Another disadvantage of this optical connector is that it is possible for certain components of the optical connector to be improperly assembled during the assembly process. This can be seen with reference to FIG. 2 of the '752 patent. A cable retention member is adapted to receive a barrel and spring of the ferrule assembly during the assembly process. The cable retention member includes a collar which is chamfered such that when the cable retention member is inserted within the plug frame, the side portions of the collar are received within windows of the plug frame. However, the plug frame has a cylindrical, or annular, opening that does not include any type of keying mechanism for ensuring that the side portions of the collar are received within the windows of the plug frame. Consequently, it is possible for the cable retention member to be pressed into the plug frame in such a manner that the side portions of the collar do not align with the windows. However, even if the side portions of the collar do not align with the windows, the cable retention member will be locked into place within the plug frame via a friction fit that makes it difficult, if not impossible, for the cable retention member to be removed from the plug frame. Therefore, improper assembly of the optical connector is possible if measures are not taken to ensure proper alignment of the cable retention member with the plug frame during assembly.
The improper assembly of the cable retention member within the plug frame prevents the optical connector from having a side-loading capacity that is as great as it would be if the side portions of the collar were properly seated within the windows of the plug frame. Also, once the cable retention member has been improperly inserted into the plug frame, it is difficult, if not impossible, to properly couple the plug frame with the grip, which will make it difficult, if not impossible, to couple the optical connector to the coupling housing in order to enable the ends of two optical fibers to be optically coupled together.
Another known design of an optical connector is shown in U.S. Pat. No. 5,481,634 (hereinafter the '634 patent). This connector utilizes a two-piece housing assembly comprising a housing and a cover, which are ultrasonically bonded together after a ferrule and its associated components have been installed within the housing. The associated components comprise a fiber-holding structure that includes the ferrule, a base member and a spring that is disposed about the base member. The housing is a generally U-shaped device having a cavity for receiving the fiber-holding structure. Once the fiber-holding structure has been inserted into the cavity of the housing, the cover is bonded thereto. The cover includes pins that mate with holes in the housing for alignment. Once joined together by the pins and associated holes, the front end of the connector has a generally square shape that fits into a receptacle that is shaped to receive the connector. The connector has a spring latch molded thereto that includes a living hinge, which allows a tab to be moved up and down in a direction that is generally perpendicular to the axial passageway of the fiber-holding structure. The spring latch is used for securing the connector to the receptacle in order to prevent unintended decoupling of the connector and the receptacle.
The housings of optical connectors, such as those discussed above, are often comprised of either polycarbonate or polyetherimide (PEI). For example, the '752 patent discloses that the plug frame of the connector may be comprised of polycarbonate. Both of these polymers have certain desirable properties. For example, polycarbonate is a relatively strong material that has good side-loading characteristics. Side-loading is usually applied by pulling the cable behind the connector; testing is typically done at 90.degree. to the fiber axis. Optical connectors must withstand at least a certain minimum amount of side-loading in order to operate properly. As is well known in the art, when optical fibers are bent beyond a particular bending radius, undesirable signal loss or attenuation occurs. Therefore, an optical connector needs sufficient side-loading capability in order to prevent the optical fibers housed therein from being bent beyond an allowable bending radius. Optical connectors comprised of polycarbonate tend to have relatively good side-loading characteristics. Polycarbonate is also relatively inexpensive, which is also an advantage of using polycarbonate with optical connectors.
Although PEI is stronger than polycarbonate, polycarbonate is more flexible than PEI. Flexibility is an important and desirable property because it can enhance the life of the connector, or of particular features of the connector. For example, if the portion of the connector disclosed in the '634 patent having the living latch thereon and the latch itself were comprised of PEI, the latch, when bent a number of times, will fracture sooner than if it were comprised of polycarbonate. Since latches are intended to be flexed in order to decouple the connector from an associated receptacle or adapter, the ability of the latch to be flexed a number of times without breaking is important. However, there is another advantage of using PEI for the connector; it has superior chemical resistance to anaerobic adhesives and primers, which are often used to attach an optical fiber to the ferrule in private networks; and PEI has superior environmental performance (e.g., temperature and humidity tolerance) compared to polycarbonate.
Prior art connectors have also used two-piece housings wherein the extender cap and plug body are snapped together mid-span. In this design, PEI housings have been used due to the chemical resistance of PEI and its superior strength performance. However, as noted earlier, PEI has certain undesirable properties, such as stiffness and lower flexibility, which are undesirable for the latch feature. Polycarbonate was not chosen for use with this design because of its poor chemical resistance to anaerobic adhesives and primers, although the associated flexibility of a latch made of polycarbonate would have been acceptable.
Since PEI is less flexible and stronger than polycarbonate, it has greater side-loading capability than polycarbonate, but it is more susceptible than polycarbonate to fracturing or breakage due to bending or flexing. Therefore, if the living latch disclosed in the '634 patent were made of PEI, the latch could not be bent or flexed as many times as a latch comprised of polycarbonate before breaking. For these and other reasons, polycarbonate generally is not suitable for use with optical connectors. Polycarbonate is, however, widely used in connection with telephone jacks, which is an area of technology in which anaerobic adhesives are normally not used.
Other desirable properties of materials that are used for making optical connectors include low sensitivity to molding parameter variations and the ability of the material to knit well. When the housings of optical connectors are molded, the material of which they are molded typically flows into the mold cavity around and through various gaps and paths and rejoins. Rejoining of the plastic flow fronts is commonly referred to as knitting. At locations where the material rejoins, seams are sometimes formed as the material solidifies. These seams can be susceptible to stress such that stress applied to the knit line can result in fracture. Therefore, it is important that the optical connector be comprised of a material that has good knitting properties. PEI generally has relatively poor knitting properties, which, of course, is an undesirable property of PEI in relation to its use with optical connectors.
If a material has a high sensitivity to molding parameters, e.g., the temperature of the mold, the temperature of the melt, the speed at which the mold is filled, etc., then a failure to adequately control one or more of the molding parameters will likely result in poor quality of the end product. Therefore, it is desirable to make optical connectors out of a material that has a relatively low sensitivity to molding parameters so that variations in one or more of the molding parameters outside of their optimum values, or ranges of values, will not necessarily result in an end product of poor quality.
Accordingly, a need exists for an optical connector that is comprised of a material that provides the connector with good side-loading and flexibility characteristics, good chemical resistance, good environmental performance over a wide range of temperature and humidity ranges, and that has desirable knitting properties and relatively low sensitivity to molding parameters.